Gas evolution from glasses has been studied both from th standpoints of refining of glass and the creation of glass foams. As an example of a glass foam, a commercial material marketed under the trademark FOAMGLAS by Pittsburgh-Corning, Pittsburgh, Pa. is produced in large volume by melting a typical soda-lime glass under highly oxidizing conditions (utilizing Na.sub.2 SO.sub.4 in the batch), comminuting the glass to a fine particle size, and firing the glass particles in combination with powdered carbon. A coarse foamed glass of low density (&lt;0.2 g/cm.sup.3) is formed containing carbon dioxide bubbles of several millimeters' diameter. The glass is gray or black in color with a porous and dull surface.
Porous glasses, glass-ceramics, and sintered ceramics have frequently been described in the patent literature. Examples of such disclosures include:
U.S. Pat. No. 1,108,007 is directed to the melting of basalt in graphite crucibles. The molten basalt reacts with the graphite and bubbles of a gas (not identified) are entrained in the melt. Upon cooling, the melt crystallizes to a porous body.
U.S. Pat. No. 2,978,340 is concerned with the preparation of hollow glass spheres from discrete, solid particles consisting of an alkali metal silicate, e.g., sodium silicate, a metal oxide which forms an insoluble glass when melted with the silicate, e.g., B.sub.2 O.sub.3, and a bloating agent. An extensive list of gasifying agents is furnished, none of which is singled out as exhibiting any unusual behavior.
U.S. Pat. No. 3,189,512 is drawn to foamable ceramic cements wherein a combination of SiC and SO.sub.3 comprised the foaming agent. The cements were composed of PbO, a metal fluoride, SiC, SO.sub.3, and a lithium aluminosilicate material (conveniently petalite).
U.S. Pat. No. 3,261,696 reports a method for forming insulating foamed materials comprising the steps of: (a) combining ZrO.sub.2, Al.sub.2 O.sub.3, and powdered aluminum; (b) adding H.sub.3 PO.sub.4 to the mixture to cause a reaction to occur which liberates water vapor and hydrogen to foam the mass; and (c) curing the mass at 150.degree.-800.degree. F.
U.S. Pat. No. 3,634,111 discusses foamable ceramic cements. The cements consisted of a glass having a composition within the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 system containing SiC as the cellulating agent, and being essentially free from PbO, SO.sub.3, and fluoride.
U.S. Pat. No. 3,811,852 discloses the preparation of porous glass-ceramic masses comprising the steps of frothing the initial glass melt with gas liberated through fuel combustion in the melt, forming glass ribbon from the melt, and thereafter heat treating the glass ribbon in a two-step process to convert the glass into a glass-ceramic.
U.S. Pat. No. 4,011,093 describes a foamable ceramic cement consisting essentially of a glass frit having a composition within the Li.sub.2 O--Al.sub.2 O.sub.3 --CeO.sub.2 --SiO.sub.2 system with, optionally, ZnO into which SiC is incorporated as a foaming agent.
U.S. Pat. No. 4,084,980 is drawn to the production of a foamed body comprising the steps of: (a) mixing the following four components, viz., an aqueous solution of an acid or a water soluble acidic phosphate, a cement material or an anhydrous alkali metal silicate, a metal blowing agent, and a foaming stabilizer, to obtain a pasty mass; (b) shaping the pasty mass into a desired geometry; and (c) allowing the shaped mass to stand to effect foaming.
U.S. Pat. No. 4,116,703 is directed to the preparation of a foamable cement which comprises mixing together crystalline hydraulic cement, a hydraulic cement in the form of a silicate glass powder, and quaternary ammonium silicate, and then allowing the mixture to react and set at a temperature below 150.degree. C.
U.S. Pat. No. 4,133,691 is concerned with the development of an inorganic foam which comprises the steps of: (a) mixing particulate aluminum with an aqueous solution of an alkali metal base to cause the formation of hydrogen gas; (b) folding that mixture into an aqueous alkali metal silicate solution in a manner to retain concentrated areas of the mixture in the silicate solution; and (c) thoroughly mixing the materials to form a solid foam.
U.S. Pat. No. 4,404,291 reports a method for forming a molded sintered porous body comprising the following steps: (a) mixing a powdered organic combustible material with powdered glass, devitrifying solder glass, or glass-ceramic; (b) heating the mixture to a temperature sufficient to burn off the organic material to form open pores in the resultant mass; and then (c) heating the mass to a temperature sufficient to sinter the powders together into an integral body.
As can be observed from the above, various mechanisms have been employed to prepare foamed glass and glass-ceramic bodies. Nevertheless, the production of foamed glasses and glass-ceramics exhibiting the highly desirable combination of fine bubble size, low density, and a non-porous surface has not been satisfactorily achieved. Hence, the primary objective of the present invention is to provide such products. cl SUMMARY OF THE INVENTION
The basis of the instant invention is the finding that foamed, closed-pore glass and glass-ceramic articles, wherein very fine bubbles composed predominantly of hydrogen are present, can be formed over a range of compositions in the following fundamental systems; viz., SiO.sub.2 --Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --RO--R.sub.2 O, P.sub.2 O.sub.5 --SiO.sub.2 --B.sub.2 O.sub.3 --[RO], and SiO.sub.2 --Al.sub.2 O.sub.3 (B.sub.2 O.sub.3)--P.sub.2 O.sub.5 --Li.sub.2 O--[ZrO.sub.2 (TiO.sub.2)], wherein RO is selected from the group of MgO, CaO, SrO, BaO, and ZnO, and R.sub.2 O is selected from the group of alkali metal oxides, conveniently Li.sub.2 O, Na.sub.2 O, and/or K.sub.2 O. Ammonium salts constitute the preferred source of hydrogen, although similar effects can be obtained in certain compositions through a combination of carbohydrates, hydrocarbons, and amines with phosphates.
As used herein, the term gas-ceramic indicates a body formed by a process wherein foaming concurrently accompanies crystallization; glass microfoam designates a body formed by a process wherein foaming is generated without crystallization. Gas-ceramics can be produced either through foaming by controlled nucleation of bubbles upon heat treatment of a precursor glass body or by spontaneous nucleation upon cooling of a molten glass to a solid body.
In a general composition survey of the three operable systems, three limitations appear to be unqualifiedly mandatory; viz., at least 8% by weight SiO.sub.2, at least 30% by weight B.sub.2 O.sub.3 +Al.sub.2 O.sub.3 +P.sub.2 O.sub.5, and at least 10% by weight B.sub.2 O.sub.3 +P.sub.2 O.sub.5. Although both B.sub.2 O.sub.3 and P.sub.2 O.sub.5 are desirable in combination in all composition systems, neither alone is absolutely necessary. Furthermore, all the compositions appear to be "acid"; i.e., SiO.sub.2 +B.sub.2 O.sub.3 +Al.sub.2 O.sub.3 +P.sub.2 O.sub.5 &gt;&gt;RO+R.sub.2 O, and B.sub.2 O.sub.3 +Al.sub.2 O.sub.3 +P.sub.2 O.sub.5 &gt;RO+R.sub.2 O. Fluorides appear to be undesirable, as are most easily reducible metal oxides, although sometimes minor amounts of TiO.sub.2 and rather considerable levels of ZnO can be tolerated.
The most effective batch ingredients for the introduction of hydrogen-forming species include NH.sub.4 H.sub.2 PO.sub.4, (NH.sub.4).sub.2 HPO.sub.4, NH.sub.4 Cl, NH.sub.4 B.sub.4 O.sub.7, and starch and/or sugar with Al(PO.sub.3).sub.3. Concentrations of those materials ranging from 0.5 to over 50% of the total batch have been found effective, depending upon the base glass composition.
Foaming of the samples was accomplished by heating at about 700.degree.-1000.degree. C., depending upon the base compositions thereof, for times ranging from about 10 minutes to several hours.
The hydrogen-containing cells or bubbles are believed to be the result of either the brakdown of ammonium species in the glass, followed by dissolution of hydrogen molecules in the glass at high temperatures and subsequent release at low temperatures, or by the reduction of stable OH.sup.- ions in the glass network through reaction with a reduced phosphorus species such as P.sup.+3 or P in the glass.